1. Field of the Invention
This invention relates in general to electronic relative position sensing devices or devices which indicate the relative position of two elements, one moveable with respect to the other. More particularly, but without limitation, this invention relates to such devices having machine readable indicia on a scale fixed to one of the elements. Further, this invention relates to methods of using relative position sensing devices and reading the indicia thereon.
2. Description of the Prior Art
In many areas of industry and manufacturing, it is important to accurately position one element or device with respect to another. For example, in machining on a lathe with a movable table, it is important that the table carrying the tool bit be accurately positioned with respect to the bed holding the chuck and the work piece. To allow this positioning to be performed, a position sensor can be used. In addition to accurate relative positioning, it is also often necessary to move the table and tool bit to and from or between very accurate relative positions with respect to the bed.
For example, it might be desired to perform a first machining operation on the work piece with the tool bit at one relative position and then automatically move the tool bit (by moving the table on the bed) to a second position and perform a second machining operation on the work piece. Many times it is desired to have great accuracy in the machined work resulting from the first and second position machining operations. A position sensor connected to the table drive can be used for automatic performance of such multiple stops. Still further, it is often desirable to change the positions at which stops or operations are performed, and this requires that new position locations be established with accuracy. An electronic position sensor can allow the machine to be electronically reset to new stops.
Of course, it is desirable to provide simpler and less expensive methods and devices for performing these sensing operations more quickly and more accurately. This is especially true in fields such as integrated circuit manufacture where extremely accurate positioning is required.
Among the devices which desirably use such relative position sensing methods and devices are lathes, milling machines, microscopes, telescopes, industrial robots, cylinders, flight controls, drill presses, production automation equipment, etc.
Among the devices currently available for relative position sensing are electronic relative position sensors which have scales provided on them and means for electrically reading these scales. The scale can be attached to the moveable table carrying the tool bit or the like, and the electrical reader of the scale can be connected to the lathe bed or the like, or vice versa. As the device moves, the reader electrically senses or determines the relative position between the scale and reader and, thereby, the relative position between the tool bit table and lathe bed and the devices to which they are attached.
In general, electronic relative position sensing devices fall into two major categories. The first category is best characterized as a serial device or counter. These devices are only capable of determining the distance between two stops by counting or otherwise electrically measuring the distance between the two stops. Because the counter has no means of sensing where on the scale it is because it can only measure by counting, this device measures only relative positions. The second kind of device is best characterized as an absolute position sensing device. In these devices, where on the scale ----an absolute position---- is able to be determined and this allows relative positions to also be determined. In other words, a position on the scale, such as 3 inches from the zero point of the scale, is readable directly from absolute position indicia on the scale at that point.
Prior to this invention, the serial devices or counters have been most successful because of greater accuracy and lower cost. Among these devices are rotary counters which have a wheel, gear or screw which turns as a result of the motion and an electronic counter which counts rotations or parts of rotations of the wheel or screw. Similarly, there are devices which have magnetic marks or slits or other indicia on a bar and provide an electronic means for counting the passage of each one of these indicia. Of course, in both of these devices, the accuracy of the location of the machine-read indicia on the scale determines the accuracy of the device. Thus, the best accuracy of these devices is approximately 20 microinches (0.00002 inches).
A good example of the most accurate of the serial or incremental position sensing devices are Heidenhain electronic measuring scales made by Heidenhain Corporation. These devices have glass scales with extremely accurate etched markings positioned thereon which can be electronically counted by a movable reader. To accurately etch the markings requires a laboratory secured deep underground with local trains prevented from running to avoid distant earth vibrations. Isolation tables, unique machine tools and unique measuring equipment are required to perform the etched marking. Of course, the scales produced in this way so as to provide 20 microinch accuracy are very expensive.
Position sensing rotary devices, like linear counters, count increments to determine position. Rotary devices, however, count turns or portions of turns of a position wheel. A problem with these devices is that they lose accuracy if there is any slippage between the position wheel and the device causing the rotation and gears do not move entirely smoothly as they rotate. Both the linear and the rotary counters are unable to determine where on the scale the device is if the power is turned off or is otherwise interrupted so that it loses its place.
Absolute position devices avoid the problem of losing their place by having place determining markings thereon. A good example of such a device is shown in U.S. Pat. No. 4,074,258 to Dore, et al. This device includes a graduated scale which carries marks which are transparent on an opaque background or opaque on a transparent background so the marks can be interpreted by a photo-electric reader. A first set of identical marks are distributed at equal intervals along the scale, for example, at one inch intervals. These marks are precisely located because the location of each mark is used as a reference for interpolation. In this regard, the scale is like a serial positioning device.
However, adjacent the accurately positioned and spaced interpolation marks are a group of identifying marks (typically a binary number representation) which can be electrically determined as uniquely identifying the adjacent precisely located interpolation mark. The interpolation marks are distinguished from the identification marks by the intensity of light, by the width of the marks or by other means. In this way the electrical interpretation of the identifying marks can be made allowing the particular location of the interpolation mark to be identified. Then a characteristic such as the leading edge of the interpolation mark is used to provide a relative distance along the reader.
In one embodiment, the Dore et al. patent describes distinguishing between interpolation marks and identifying marks by the width of the marks as read by light on a charge-coupled device. An interpolation mark is five pixels wide and an identifying mark is 10 pixels wide. Specifically, a first pixel of a five pixel interpolation mark, identified by the binary code of 10 pixel identifying marks adjacent thereto, is read from the pixels of a charge-coupled device reader as the relative position of the CCD reader with respect to the scale.
Of course, a problem with this device is that the identifying marks cause the position or interpolation marks to be farther apart. This means the accuracy of the device is reduced because less magnification (the means by which the marks are more finely interpreted) is possible.
Other similar absolute position devices utilize marks of identification located transversely to the interpolation marks or position marks. The difference in position allows these marks to be separately read so as to distinguish them without differences in width or intensity. With these devices, however, a separate reader or readers is required for the identifying marks.
In general, absolute and relative devices suffer by not being sufficiently accurate. Like relative devices, the precision of an absolute device depends upon the precision with which the interpolation or position marks are placed on a scale. Very accurate painting, grinding, cutting or etching is required and the limits of this marking sets the limit for the accuracy of the device. Of course, the most accurate of these devices are very expensive because of the extreme manufacturing techniques required.
Another problem with the absolute positioning devices is that the means for reading the identifying marks are complicated, slow, and inaccurate. Thus, an electronic means must be provided to distinguish the interpolation marks from the identifying marks, the identifying marks must then be converted to a form which can be read and then the binary number or the like must be associated with the proper interpolation or positioning mark.
The position of the identifying mark is made using only a single characteristic of the mark such as the leading edge (first pixel of five pixel mark, for example). This leading edge is identified by the pixel of a CCD having an output signal of a predetermined strength. Variations in light source strength, electronic noise, dirt on the magnifying lens, and many other disturbing factors can prevent proper interpretation of the identifying mark position characteristic from being read or from being read accurately.
Another problem with all of these devices is that the precision of the device can be reduced or destroyed by a mounting which stresses or moves the marks with respect to each other. Still further, thermal changes cause the marks to move with respect to each other also changing the accuracy of the scale. Usually, however, the scale is not sufficiently accurate to be affected by thermal expansion. Even further, the devices for shining light through lenses or the like have not been satisfactory enough to provide accurate reading.